Electrical transformer

ABSTRACT

A transformer having a casing, a liquid dielectric in the casing, and an electrical winding in the casing having at least two circuits protected by a circuit breaker. The circuit breaker has contacts in each protected circuit, with the contacts in each circuit being electrically connected to a different pair of parallel-connected, like thermal responsive elements. Only one thermal-responsive element of each pair is operative to trip the circuit breaker.

United States Patent Inventors John J. Astlelord, Jr. Sharon; William J.Willis, Sharpsville, both of Pa. Appl. No. 887,706 Filed Dec. 23, 1969Patented Oct. 5,1971 Assignee Westinghouse Electric Corp.

Plttsburgh, Pa.

ELMI'IRICAI. TRANSFORMER 7 (Tlulmu, 2 Drawing FIRM.

317/40 R, 317/46, 337/95 Int. Cl "02h 7/04 Field of Search 317/14, 15,

[56] References Cited UNITED STATES PATENTS 3,152,287 10/1964 Edmunds337/38 X 3,398,323 8/1968 Anderson 317/15 Primary Examiner-James D.Trammell Attorneys-A. T. Stratton, Donald R. Lackey and F. E.

Browder ABSTRACT: A transformer having a casing, a liquid dielectric inthe casing, and an electrical winding in the casing having at least twocircuits protected by a circuit breaker. The circuit breaker hascontacts in each protected circuit, with the contacls in each circuitbeing electrically connected to a different pair of parallel-connected,like thermal responsive elements. Only one thermal-responsive element ofeach pair is operative to trip the circuit breaker.

BACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates in general to electrical transformers, and more specifically toelectrical transformers of the distribution type which include a circuitbreaker.

2. Description of the Prior Art Electrical distribution transformersoften include a circuit breaker within their casing, for protecting thetransformer against predetermined overload conditions in the secondaryor load circuit. The circuit breaker hascontacts connected in eachsecondary circuit of the transformer to be protected, with a thermallyresponsive element, such as a bimetal or trimetal, being located andconnected to be responsive to both the temperature of the insulatingfluid disposed in the casing, and the magnitude of the current flowingin the protected circuit.

The circuit breakers used in distribution transformers are made up ofsingle-pole assemblies, assembled and interlocked in gangs of two orthree, for single or three-phase operation, respectively. Each poleassembly has a molded, insulating case or housing, with the poles beingavailable in different frame sizes and electrical ranges to accommodatethe different kv.-a. ratings of distribution transformers. Since thecost per breaker pole increases with its electrical rating, and sincethe physical size of the thermally responsive element is the firstfactor which necessitates going to a larger frame breaker, rather thanthe current capability of the contacts, various arrangements have beenused in the prior art to subject the thermally responsive element toonly a predetermined portion of the total secondary current, whileconnecting the contacts of the circuit breaker to interrupt the totalsecondary current, instead of only the portion flowing through thethermally responsive element. This reduces the size of the thermallyresponsive element required, and enables smaller and less costly circuitbreaker pole assemblies to be assembled to protect a transformer.

All of the prior art arrangements for reducing the amount of secondarycurrent flowing through the thermally responsive element, whileinterrupting the total secondary current with the circuit breakercontacts, have certain disadvantages. For example, providing a conductorin shunt with a thermally responsive element is, in general,undesirable, as the differences in the electrical impedances of the twopaths provide a large unbalance in the current division. Adding aresistor of predetermined value to the shunt circuit about the thermallyresponsive element, to match the impedance of the thermally responsiveelement, is costly, as the resistor required is neither a standardbreaker nor a standard transformer element, and it adds to the assemblycost of the transformer as special mounting and insulating hardware isrequired.

Providing a two-part or split winding for each section of thetransformer secondary, with both parts being connected to the breakercontacts, and only one part going through a thermally responsiveelement, such as disclosed in US. Pat. No. 2,597,185, which is assignedto the same assignee as the present application, reduces the currentunbalance, but has the disadvantage of the additional cost of providingsplit windings.

Therefore, it would be desirable to be able to controllably andpredictably reduce the amount of secondary current flowing through athermally responsive element of a circuit breaker associated with adistribution transformer, while interrupting the total secondary currentwith the breaker contacts, without resorting to split winding sectionsand/or shunt resistors which are nonstandard to both circuit breakersand transformers, requiring special mounting and insulating hardware.

SUMMARY OF THE INVENTION Briefly, the present invention is a new andimproved transformer having a circuit breaker disposed within itscasing, which for each secondary circuit to be protected includes a pairof parallel-connected, like thermal responsive elements, and breakercontact means. The breaker contact means is connected in series with apair of parallel-connected thermally responsive elements in a secondarycircuit to be protected.

The pair of thermally responsive elements for each circuit to beprotected is provided by utilizing two circuit breaker pole assemblies,each having a thermally responsive element, but only one of which hasbreakercontacts and associated operating mechanism. Thus, one poleassembly is a conventional or active" pole, and the other is adummyflpole, with the dummy pole supporting the thermally responsiveelement, insulating the element and enabling it to be mounted in aconventional manner. Since the same thermally responsive element is usedin each branch of the parallel circuit, the current divides in asubstantially equal manner without resorting to split secondary windingsections, allowing thermally responsive elements of one-half the normalrating to be used. Further, the dummy and active pole assemblies areassembled using standard circuit breaker items, enabling them to beganged together in a conventional manner and mounted without specialhardware and mounting procedures.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of theinvention will become more apparent when considered in view of thefollowing detailed description and drawings, in which:

FIG. 1 is a schematic diagram of a transformer and circuit breakerarrangement constructed according to the teachings of the invention; and

FIG. 2 is a partially schematic and partially perspective view of atransformer and circuit breaker arrangement constructed according to theteachings of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, andFIG. 1 in particular, there is shown a transformer 10, having a casingshown generally at 12, which is filled to a predetermined level with afluid insulating and cooling dielectric, such as mineral oil.Transformer [0 includes a core-coil assembly 14 disposed within casing12 and immersed in the liquid dielectric, with the core-coil assembly 14having primary and secondary windings l6 and 18, respectively, disposedin inductive relation with a magnetic core 20. The secondary winding 18has first and second sections 22 and 24, respectively, which areconnected serially at junction 26.

The primary winding 16 is connected to an external source of alternatingpotential (not shown) through insulating primary bushings 28 and 30, andthe secondary winding 14 is connected to a load circuit (not shown)through a circuit breaker, shown generally at 40, and insulatingsecondary bushings 32, 34 and 36. The common connection or junction 26of the first and second secondary winding portions 22 and 24 isconnected via conductor 38 to secondary insulating bushing 34, which isa ground terminal, and the other ends of the first and second windingportions are connected through the circuit breaker 40 to insulatingbushings 32 and 36, respectively.

Each of the secondary circuits to be protected, such as two asillustrated in this embodiment, includes contact means of a pair or setof contacts from circuit breaker 40, such as one of the contact pairs 42and 44. One end of the first secondary winding portion 22 is connectedto one of the contacts of pair 42, via conductor 46, such as thestationary contact of the pair, and one end of the second secondarywinding portion 24 is connected to one of the contacts of pair 44 viaconductor 48, such as the stationary contact of the pair.

The remaining contact of each pair is connected to a secondaryinsulating bushing through a thermally responsive element, such as abimetal, which bimetal is operative to effect the actuation of thecircuit breaker 40. More specifically, the remaining contact of pair 42,such as the movable contact, is connected to one end of a thermalresponsive element 50, such as the movable end of the element, and theother end, such as the fixed end of the element, is connected toinsulating bushing 32 via conductor 52. In like manner, the remainingcontact of pair 44, such as the movable contact, is connected to one endof a thermally responsive element 54, and the other end is connected toinsulating bushing 36 via conductor 56.

Up to this point, the described construction requires two standardcircuit breaker pole assemblies, shown generally at 60 and 62, whichpole assemblies are interlocked mechanically to operate together. Thepoles may be used with, or without magnetic trips (not shown), asdesired. The thermally responsive elements 50 and 54 of breaker 40 aredisposed in the insulating fluid of the transformer and are thermallyactuated load-responsive elements for initiating the operation ofcircuit breaker 40 to disconnect the transformer secondary winding 18from an external load circuit upon predetermined overload conditions inthe secondary circuit. The operation of either thermal responsiveelement 50 or 54 will trip both poles 60 and 62 of the breaker 40,through trip means and the mechanical interlock. The thermallyresponsive elements are responsive to the temperature of the transformercooling fluid, and also to the magnitude of the current flowing in thetransformer secondary circuit.

However, with the arrangement described to this point, the totalsecondary current will flow through the thermally responsive elements.For example, if the transformer 10 is rated 100 kv.-a., the thermallyresponsive elements 50 and 54 must also each be rated 100 kv.-a.

This invention teaches how the ratings of the thermally responsiveelements may be reduced by one-half, dividing the current flow into twosubstantially equal portions in each circuit to be protected, with athermally responsive element being disposed to be responsive to only thecurrent in one of the portions. Thus, the rating of the thermallyresponsive elements 50 and 54 may be reduced by one-half, which reducesthephysical size of the thermally responsive element and enables smallerand less costly circuit breaker pole assemblies to be utilized. Further,this result is accomplished without splitting each secondary windingsection into two parts, and without introducing nonstandard parts withcostly insulating and mounting hardware.

Specifically, each thermally responsive element 50 and 54 has athermally responsive element of like rating connected in shunttherewith, using the same length and size of electrical leads for theshunt element as used to connect elements 50 and 54 into the protectedcircuits. Thermally responsive element 50 has a thermally responsiveelement 70 of like rating connected in shunt therewith, providing aparallel-connected pair of thermally responsive elements between contactpair 42 and secondary terminal 32, with each path of the parallelcircuit having similar leads of like cross section and length, andsimilarly rated thermally responsive elements, i.e., bimetals ortrimetals, to provide a substantially equal division of current througheach branch of the parallel circuit, eliminating the necessity of splitsecondary winding sections. In like manner, thermally responsive element54 has a thermally responsive element 72 of like rating connected inshunt therewith, providing a parallel-connected pair of thermallyresponsive elements between contact pair 44 and secondary terminal 36,which also provides a substantially equal division in the two branchesof the parallel circuit.

Since the thermally responsive elements used to shunt the thermallyresponsive elements actively associated with contact pairs 42 and 44 aresimilar, they are standard, readily available items which are stockedalong with the connecting leads of the proper size and length. Further,the use of like thermally responsive elements to reduce by one-half thecurrent flow through the active thermally responsive elements 50 and 54enables certain economies in both the assembly of the additionalthermally responsive elements, and in the mounting of them in thetransformer, to be realized, as breaker pole assemblies may be assembledwhich include only a thermally responsive element, eliminating thebreaker contact pair and operating mechanism. The molded insulating caseor housing of the additional breaker pole assemblies, shown generally atand 82, may be ganged with pole assemblies 60 and 62, i.e., theirhousings are disposed in aligned, contacting, side-by-side relation,thus assembling and mounting the additional thermally responsiveelements in a conventional manner, which eliminates special mounting andinsulating hardware, as well as special mounting procedures. The breakerpole housing supports and insulates the thermally responsive element,and yet the housing without the associated contact pair and operatingmechanism has a relatively low manufacturing cost due to the highproduction of this item.

The advantages of using thermally responsive elements of like rating toshunt thermally responsive elements which actuate the operatingmechanism of the circuit breaker may be more readily appreciated byexamining FIG. 2, which is a partially schematic, partially perspectiveview of the transformer 10 shown in FIG. I. Like reference numerals inFIGS. 1 and 2 refer to like components.

Specifically, FIG. 2 illustrates circuit breaker 40 in an explodedperspective view, with pole assemblies 80 and 82 being of similarconstruction, having thermally responsive elements, such as bimetals 70and 72 mounted in insulating housings 92 and 94, respectively. Theremaining pole assemblies 60 and 62 are of like construction, havinginsulating housings 96 and 98, respectively, which are of the same framesize as housings 92 and 94 of poles 80 and 82, respectively, enablingthem to all be ganged together for mounting within the transformer. Inaddition to thermally responsive elements, such as bimetals 50 and 54,breaker poles 60 and 62 each have contact pairs 42 and 44, respectively,and an operating or trip mechanism. For example, pole 60 has anoperating mechanism which includes a main breaker latch 100, latchspring 102, a catch 104 on the bimetal 50 for engaging latch 100, a triparm I06, a contact arm 108 which carries the movable contact of pair 42,a toggle link 110, a toggle spring 112, a contact opening spring 114 anda handle (not shown) interlocked with handle 116 of pole 62, viaconnecting pin 118. The operating mechanism of pole 60, is shown merelyfor purposes of example, as any suitable breaker-operating mechanism maybe used. Since the operation of the breaker mechanism, for closing andopening the set of contacts, is well known in the art, such as disclosedin US. Pat. No. 2,686,242, which is assigned to the same assignee as thepresent application, the details of the breaker operation will not bedescribed.

The stationary contact of pair 42 of pole 60 is connected to a flexiblelead 120, which is connected to the secondary winding 18, and thecircuit proceeds through contact pair 42 to a parallel circuit whichincludes its own associated bimetal 50, via leads 122, and then viasimilar leads, shown generally at 124, to bimetal 70 of adjacent pole80. Bimetals 50 and 70 include flexible leads I26 and 128, respectively,which are connected together and to the secondary insulating bushing 32.

The stationary contact of pair 44 of pole 62 is connected to a flexiblelead 130, which is connected to secondary winding 18, and the circuitproceeds through the contact pair 44 to a parallel circuit whichincludes its own associated bimetal 54, via leads 132 and also viasimilar leads, shown generally at 134, to bimetal 72 of adjacent pole82. Bimetals 54 and 72 include flexible leads 136 and 138, respectively,which are connected together and to the secondary insulating bushing 36.

Poles 80, 60, 62 and 82 are shown arranged in the preferred order, asthe two active poles may be most conveniently mechanically interlockedif mounted adjacent one another. but any other order may be used whereinthe bimetals to be connected in parallel are adjacent one another, asthe dummy poles do not have handles which would interfere with aninterlocking pin between the two active poles.

The disclosed transformer and circuit breaker arrangement 10, has adistinct advantage over using four active poles ar ranged as illustratedin FIG. 2, beyond the obvious cost savings realized by eliminating theactive elements of two of the poles. For example, if four active poleswere to be used, the differences in the contact resistances from pole topole would be such that substantially equal current division would nolonger be obtainable, making it necessary to resort to split secondarywinding sections, adding still further to the manufacturing cost of theapparatus. Further, four active poles would be difficult to coordinate,as the mechanical load on the trip latch would be double thatexperienced when using the teachings of the invention. Latch spring 102may not be sufficient to trip four poles, and if a stronger spring isresorted to it would change the thermal sensitivity of breaker 40, asthe friction between the bimetal catch 104 and latch 100 would begreater in each of the poles.

While the invention has been described relative to a singlephasetransfonner having two secondary circuits to be protected, it will beobvious from the foregoing disclosure that a three-phase transfonnerhaving three secondary circuits may be protected in a similar manner, aseach secondary circuit to be protected includes a pair ofparallel-connected, like thermally responsive elements connected inseries with contact means, with only one of the thermally responsiveelements in each circuit to be protected being operative to trip thecircuit breaker.

In summary, there has been disclosed a new and improved transformer andcircuit breaker arrangement which enables thermally responsive elementsto be used having one-half the current rating of the transformer circuitto be protected, without resorting to split secondary winding sectionsand/or resistive shunt elements which are not standard to either thecircuit breaker or the transformer, and which must be speciallyinsulated and mounted with nonstandard hardware. The disclosedarrangement utilizes only standard circuit breaker and transformercomponents already used in circuit breaker protected transfonners,simplifying the stocking requirements and the assembly and the mountingof the circuit breaker within the transformer, and it obtains asubstantially equal division of current between an active thermallyresponsive element and a circuit in shunt therewith, in each secondarycircuit of the transformer to be protected.

Since numerous changes may be made in the abovedescribed apparatus anddifferent embodiments of the invention may be made without departingfrom the spirit thereof, it is intended that all matter contained in theforegoing description or shown in the accompanying drawings shall beinterpreted as illustrative, and not in a limiting sense.

We claim as our invention: 1. A transformer comprising: a casing, acircuit breaker disposed in said casing, electrical winding meansdisposed in said casing, having at least one circuit to be protected bysaid circuit breaker,

said circuit breaker including a pair of parallel-connected, likethermally responsive elements, connected in series with contact means,in each of the circuits to be protected, and means for tripping thecircuit breaker,

only one of the thermally responsive elements in each circuit to beprotected being operative to actuate said means for tripping saidcircuit breaker.

2. The transformer of claim 1 wherein the electrical winding means is asingle-phase secondary winding having first and second circuits to beprotected.

3. The transformer of claim 2 wherein the circuit breaker includesfirst, second, third and fourth pole assemblies, each including aninsulating housing and a thermally responsive element, and with only thesecond and third poles having contact means.

4. The transformer of claim 3 wherein the insulating housings of first,second, third and fourth pole assemblies are disposed in side-by-siderelation, res ctivel and including means interlocking the second and tird p0 e assemblies, to open both contact means when either is trippedby its associated thermally responsive element and the means fortripping the circuit breaker.

5. The transformer of claim 3 wherein the thermally responsive elementsof the first and second pole assemblies are connected in parallel, andthe thermally responsive elements of the third and fourth poleassemblies are connected in parallel, with the parallel-connectedthermally responsive elements of the first and second poles beingconnected in series with the contact means of the second pole assembly,in the first circuit of the secondary winding, and theparallel-connected thermally responsive elements of the third and fourthpole assemblies are connected in series with the contact means of thethird pole assembly, in the second circuit of the secondary winding.

6. A transformer comprising:

a casing;

electrical winding means disposed in said casing; said electricalwinding means having at least two electrical circuits to be protected;

a circuit breaker disposed in said casing; said circuit breakerincluding first and second pole assemblies for each electrical circuitof said winding means to be protected; said first and second poleassemblies each having an insulating housing and a thermally responsiveelement; only the first pole assembly for each electrical circuit to beprotected including a set of electrical contacts, means for opening andclosing the set of electrical contacts, and trip means disposed to openthe set of electrical contacts in response to predetermined movement ofits associated thermally responsive element;

and means connecting the thermally responsive elements of the first andsecond pole assemblies, and the set of electrical contacts of the firstpole assembly, in each circuit to be protected, with the thermallyresponsive elements being in series with the set of electrical contacts,and in parallel with one another.

7. The transformer of claim 6 wherein all of the pole assemblies for theelectrical circuits to be protected have their insulating housingsdisposed in aligned side-by-side relation, and including interlock meansdisposed to open the set of electrical contacts of all of the first-poleassemblies, upon the opening of any of the set of electrical contacts.

1. A transformer comprising: a casing, a circuit breaker disposed insaid casing, electrical winding means disposed in said casing, having atleast one circuit to be protected by said circuit breaker, said circuitbreaker including a pair of parallel-connected, like thermallyresponsive elements, connected in series with contact means, in each ofthe circuits to be protected, and means for tripping the circuitbreaker, only one of the thermally responsive elements in each circuitto be protected being operative to actuate said means for tripping saidcircuit breaker.
 2. The transformer of claim 1 wherein the electricalwinding means is a single-phase secondary winding having first andsecond circuits to be protected.
 3. The transformer of claim 2 whereinthe circuit breaker includes first, second, third and fourth poleassemblies, each including an insulating housing and a thermallyresponsive element, and with only the second and third poles havingcontact means.
 4. The transformer of claim 3 wherein the insulatinghousings of first, second, third and fourth pole assemblies are disposedin side-by-side relation, respectively, and including means interlockingthe second and third pole assemblies, to open both contact means wheneither is tripped by its associated thermally responsive element and themeans for tripping the circuit breaker.
 5. The transformer of claim 3wherein the thermally responsive elements of the first and second poleassemblies are connected in parallel, and the thermally responsiveelements of the third and fourth pole assemblies are connected inparallel, with the parallel-connected thermally responsive elements ofthe first and second poles being connected in series with the contactmeans of the second pole assembly, in the first circuit of the secondarywinding, and the parallel-connected thermally responsive elements of thethird and fourth pole assemblies are connected in series with thecontact means of the third pole assembly, in the second circuit of thesecondary winding.
 6. A transformer comprising: a casing; electricalwinding means disposed in said casing; said electrical winding meanshaving at least two electrical circuits to be protected; a circuitbreaker disposed in said casing; said circuit breaker including firstand second pole assemblies for each electrical circuit of said windingmeans to be protected; said first and second pole assemblies each havingan insulating housing and a thermally responsive element; only the firstpole assembly for each electrical circuit to be protected including aset of electrical contacts, means for opening and closing the set ofelectrical contacts, and trip means disposed to open the set ofelectrical contacts in response to predetermined movement of itsassociated thermally responsive element; and means connecting thethermally responsive elements of the first and second pole assemblies,and the set of electrical contacts of the first pole assembly, in eachcircuit to be protected, with the thermally responsive elements being inseries with the set of electrical contacts, and in parallel with oneanother.
 7. The transformer of claim 6 wherein all of the poleassemblies for the electrical circuits to be protected have theirinsulating housings disposed in aligned side-by-side relation, andincluding interlock means disposed to open the set of electricalcontacts of all of the first-pole assemblies, upon the opening of any ofthe set of electrical contacts.